Magnetic memories or magnetic random access memories (MRAM), which operate as nonvolatile memories capable of performing a high-speed operation and rewriting of an infinite number of times, have been put into practical use in specific applications and developed to improve versatility. In an MRAM, a magnetic memory element using a magnetic film is used as a memory cell and data is stored as the direction of the magnetization of the magnetic film. Typically, a magnetic memory element includes two magnetic layers and a nonmagnetic layer interposed between the magnetic layers. To read data from the magnetic memory element, a magnetoresistance effect revealed between the first and second magnetic layers, specifically, a TMR effect (tunneling magnetoresistance effect) or a GMR effect (giant magnetoresistance effect) is used.
To write data into the magnetic memory element, that is, to switch the magnetization of the magnetic film, some methods have been proposed, any of which uses a current. For practical utilization of the MRAM, it is very important how much the write current can be reduced, and according to IEEE Journal of Solid-State Circuits, vol. 42, p. 830 (2007), it is required to reduce the current down to 0.5 mA or less, more preferably, to 0.2 mA or less.
The most common method of writing data into the magnetic memory element is to arrange an interconnection near the magnetic memory element, and generate a magnetic field to switch the direction of magnetization of the magnetic film by to flow a current through this interconnection. This method theoretically enables writing within one nanosecond or less, which is suitable in realizing a high-speed MRAM. However, in a case where the magnetic memory element is configured so as to ensure thermal stability and disturbance magnetic field resistance, the magnetic field for switching the magnetization of the magnetic film is generally about a few dozens of Oe and a current of about a few mA is required to generate such a magnetic field. This undesirably increases the chip area and power consumption for writing and such MRAM is inferior to other random access memories in competitiveness. In addition, when the element is reduced in size, the write current further increases, which is undesirable also in scaling.
In recent years, the following two methods have been proposed as means for addressing such a problem. The first method is spin injection magnetization reversal. According to this method, the magnetization of the magnetic film is reversed by using spin-polarized conduction electrons. In detail, in a film stack formed of a first magnetic layer having a reversible magnetization and a second magnetic layer having a fixed magnetization, the magnetization of the first magnetic layer can be reversed due to interaction between the spin-polarized conduction electrons and localized electrons in the first magnetic layer when a current is flown between the second magnetic layer and the first magnetic layer. Since the spin injection magnetization reversal occurs at a certain current density or larger, a current necessary for writing is reduced when the magnetic memory element is reduced in size. That is, the spin injection magnetization reversal method can be said to be excellent in the scaling property. However, a dielectric layer is generally provided between the first magnetic layer and the second magnetic layer and a relatively large current must be flown through the dielectric layer in a write operation, which causes problems in terms of rewriting resistance and reliability. Moreover, since a current path in the write operation is same as that in the read operation, this may cause unintentional writing in the read operation. As described above, despite of the excellent scaling property, the spin injection magnetization reversal has some obstacles to practical utilization.
On the other hand, the second method, that is, a magnetization reversal method utilizing a current-induced domain wall motion can solve the above-mentioned problems of the spin injection magnetization reversal. The MRAM utilizing the current-induced domain wall motion is disclosed in JP2005-191032A, for example. In the MRAM utilizing the current-induced domain wall motion, the directions of the magnetization at both ends of a data recording layer having reversible magnetization are generally fixed so as to be substantially antiparallel to each other. With such magnetization arrangement, the domain wall is introduced into the data recording layer. Here, as reported in Physical Review Letters, vol. 92, number 7, p. 077205, (2004), the domain wall moves in the direction of conduction electrons when a current is flown so as to penetrate the domain wall; this allows writing by flowing a current through the first magnetic layer. Since a current-induced domain wall motion occurs at a certain current density or larger, it can be said the scaling property is excellent as in spin injection magnetization reversal. In addition, the above-mentioned problems concerning spin injection magnetization reversal can be solved in the MRAM element utilizing current-induced domain wall motion, since no write current passes through the dielectric layer and thus the write current path is separated from the read current path.
In Physical Review Letters, vol. 92, number 7, p. 077205, (2004), a current density necessary for current-induced domain wall motion is about 1×108 [A/cm2]. In this case, given that the width of a layer where domain wall motion occurs is 100 nm and the film thickness is 10 nm, the write current is 1 mA. This does not satisfy the above-mentioned requirement for the write current. Meanwhile, as described in Journal of Applied Physics, vol. 103, p. 07E718, (2008), it is reported that the write current can be sufficiently reduced by using material having perpendicular magnetic anisotropy as a ferromagnetic layer where the current-induced domain wall motion occurs. Thus, when the MRAM is manufactured so as to utilize the current-induced domain wall motion, it is preferable that ferromagnetic material having perpendicular magnetic anisotropy is used as a layer where domain wall motion occurs. Applied Physics Express, vol. 1, p. 011301 reports that current-induced domain wall motion was observed in the material having perpendicular magnetic anisotropy.
As described above, it is expected that the MRAM with reduced write current is provided by utilizing the current-induced domain wall motion occurring in material having perpendicular magnetic anisotropy.
In manufacturing the MRAM utilizing the current-induced domain wall motion in the writing method, the domain wall needs to be introduced into the data recording layer (that is, the layer where the domain wall motion occurs). In Applied Physics Express, vol. 1, p. 011301, a magnetic thin line is formed and then a part of the line is removed by etching to complete the magnetic thin with a stepped structure. Since the coercive force in the region having a smaller film thickness is smaller than that of the region having a larger film thickness in such a stepped structure, the domain wall is introduced using a magnetic field of appropriate magnitude so that the region having the smaller film thickness is reversed and the region having the larger film thickness is not reversed. Here, a magnetic field perpendicular to the substrate surface is used as the external magnetic field. Initial introduction of a domain wall into a data recording layer in a manufacturing process is hereinafter referred to as “initialization”.
However, according to such an initialization method in which a stepped structure is provided and a domain wall is introduced by using a magnetic field perpendicular to the substrate surface, the magnitude of the applied magnetic field is limited within a certain allowed range. When the allowed range is narrow, problems such as reduction in yield occur in manufacturing large-capacity magnetic memories. Therefore, it is desired that the allowed range of the external magnetic field in introducing the domain wall into the data recording layer is as large as possible.